parted: invalid token: fat

I'm trying to add a fat32 partition for /boot/efi to my router image. Here is the relevant config:

router = host hn $ props
    & hasPartitionTableType GPT
    & hasPartition
        ( partition FAT
            `mountedAt` "/boot/efi"
            `partLocation` Beginning
            `setSize` MegaBytes 10
            `setFlag` BootFlag
        )
    & hasPartition
        ( partition EXT2
            `mountedAt` "/boot"
            `partLocation` Beginning
            `setSize` MegaBytes 150
        )
    & hasPartition
        ( partition EXT4
            `mountedAt` "/"
            `addFreeSpace` MegaBytes 500
        )
    & standardSystem (Stable "stretch") X86_64
        [ dom ++ " home router" ]

But parted does not seem to support "fat" as a file system type.

parted: invalid token: fat
Error: Expecting a file system type.

It should probably use "fat32" instead, but replacing "fat" by "fat32" in the PartedVal instance of Partition.Fs fails, too:

mount: /tmp/mnteepFPo: wrong fs type, bad option, bad superblock on /dev/mapper/loop0p1, missing codepage or helper program, or other error.
umount: /tmp/mnteepFPo: not mounted.
** fatal error: failed unmounting /tmp/mnteepFPo
propellor: StopPropellorException "Cannot continue!"